1. Field of the Invention
The present invention is directed to web guides, such as web guides used to steer webs in offset printing presses.
2. State of the Art
Web guides are often used to control lateral web position and/or to correct lateral misalignment of a moving web. Commonly assigned U.S. Pat. No. 5,490,459 discloses the use of a web guide in a web printing press to laterally displace a web. U.S. Pat. No. 5,490,459 is directed to intentionally shifting the lateral web position during use of the web to wash the blanket cylinder in printing units of the press. That is, the web is shifted back and forth laterally to clean the blanket cylinder. This patent is not directed to correcting unintentional, lateral misalignments of a web. Moreover, the known guide does not perform well enough because an intentionally induced lateral web shift is not fully corrected by the web guide. It appears that known guides can correct at a high enough rate, but the shifted web has to pass through the guide area to be noticed. The error therefore remains until the shifted direction is noticed and changed.
Web guides are also known for correcting web misalignments. For example, when a new paper roll of the printing press is spliced to a previous roll, the new paper roll may not be laterally aligned with the previous paper roll. Web misalignments can also result from other sources, such as cross-web tension variations, changes in the width of new rolls spliced to the web, natural frequencies and so forth. These deviations can be both low frequency deviations and high frequency deviations. Traditionally, there are two web guides in a press system; one guide to correct web position going into the print units and one guide correcting the web position as it goes into the slitters/folder. Accordingly, a device is used to correct misalignment of the eb for proper registration with respect to print units of the press.
Conventional web guides are configured as a "box" having four idler rolls arranged such that in a cross sectional view, the four rolls are in a square or rectangular pattern. The entry and exit rolls are typically fixed in position, and two inside rolls are configured on a pivoting carriage which moves according to a desired output position of the web.
FIGS. 1A and 1B illustrate a known web guide 100 which includes downstream edge detecting sensors 102 to determine where a web 106 is positioned, laterally. FIGS. 1A and 1B illustrate a web guide control system such as that described in a brochure of MEG Inc. of East Rutherford, N.J. The correction is achieved by pivotally moving the pivoting carriage 104, which includes dual idler rolls 108, about an axis 110. The edge detecting sensors 102 are located at the exit of the web guide. A computer control including a proportional/integral/derivative (PID) regulator drives the pivoting carriage 104 about rotational axis 110. By rotating the carriage 104 about this axis in a counterclockwise direction, the web can be shifted in one lateral direction. Rotation in the clockwise direction can induce a lateral shift of the web in a second opposite lateral direction. In its direction of travel, the web passes over an idler roll 112, into pivot rolls 108 of the web guide. The web then passes from the pivot rolls 108 around another idler roll 114. The pivoting of the web to correct for lateral misalignments is performed on the pivot rolls 108. The sensors 102 detect an actual lateral misregistration of the web, and send a signal to the computer control, which then tries to correct the lateral displacement via the carriage 108.
Because the web can enter the web guide at lateral positions which will vary randomly and/or at any of several frequencies, and because there is only one desired exit position, the control system of the FIG. 1A, 1B web guide must respond to a wide range of lateral web errors. As the frequency of the positional web error increases, the web guide is less able to filter the web position to a stable condition. In addition, because the sensors 102 are located downstream of the pivoting carriage 104, web steering errors will pass through the span where they can be corrected. The correction therefore lags the actual measured error, and allows a lateral web error at the exit of the steering guide, thereby degrading product quality. Because printing press speeds have increased over the years, the problem of correcting web positional errors using a downstream sensor results in an increased amount of unacceptable product passing through the printing press.
Other similar systems known in the art are described in product brochures available from Baldwin Web Controls of Countryside, Ill., (e.g., Web Guide Control models 901/902/903), and from Web Printing Controls Company, Inc. of Lake Barrington, Ill. Although conventional web guides can correct low frequency shifting and low rate step changes (e.g., shifts on the order of about 0.002 inches/second and lower), they are unable to correct web weaving at higher frequencies and/or higher rate step changes (e.g., on the order of about 0.2 inches/second and higher).
Accordingly, it would be desirable to provide methods and systems for lateral position control of a web which overcomes the foregoing deficiencies.